Fire doors are rated in terms of the length of time a door can be exposed to furnace temperatures on one side of the door (the exposed side) without either excessive deformation of the door or excessive temperature rise on the opposite side of the door (the protected side). Additionally, the door under test is subjected to a water stream upon its exposed side immediately after the furnace temperatures are removed. These tests become increasingly rigorous as door ratings increase, i.e., the furnace temperatures become higher and the volume and force of the water stream increases for higher ratings.
Those skilled in the art recognize the fire resistance of concrete. Indeed, fireproof structures often use concrete as a basic construction material. For example, a conventional fireproof room may have concrete walls and a metal fire door, or a conventional fireproof wall may be constructed of a plurality of insulating concrete panels.
Concrete is a poor conductor of heat. Exposure of one surface of a concrete wall to a high temperature relative to the opposite surface produces a large thermal gradient through the wall. Such a thermal gradient may cause the wall to crack and, ultimately, to crumble due to uneven thermal expansion within the concrete. This effect increases as the thickness of the concrete decreases. The addition of rebar or other metal reinforcements to the wall may actually accelerate failure due to the widely differing thermal expansion coefficients of concrete and steel.
As a result of the tendency of concrete to crack when unevenly heated, fireproof doors of sizes and thicknesses for use in conventional public-building doorways tend not to be made from concrete. The ASTM (American Society for Testing and Materials) standard E152-72 for fire door testing has proven to be too rigorous for use with conventional concrete fireproof door construction techniques. Because of the severe thermal stresses induced in a door subjected to the ASTM standard, the test also determines suitability of a door for use as a security door, i.e., how well a door can resist impact damage and cracking.
For use as fire doors and security doors, the mechanical strength and weight are defining factors. Unfortunately, the stronger concrete is made, the denser it becomes and the more likely it is to crack under ASTM testing. Also, a stronger, denser concrete is likely to be too heavy for many door applications, e.g., in public buildings such as motels.
Those skilled in the art know that the addition of pumice may make concrete lighter without significantly affecting its strength. However, only insignificant reduction in weight results from the use of small-diameter aggregates, while large-diameter aggregates are unsuitable for the relatively thin panels used in doors as the aggregate diameter approaches a large percentage of door thickness.
Were a suitable concrete available, one that is both light-weight and capable of passing ASTM testing, its strength and dimensional stability would make it a desirable material for fireproof and security doors intended for public building use.